Such sample rate converters are used in digital video signal decoders, which convert sample values of the video signal digitized with a first clock frequency for the further processing of the signal, such as demodulation, decoding or digital image processing, into sample values at a second "virtual" sample frequency. They are also usable in multistandard decoders, which can process video signals of different standards with diverging color subcarrier frequencies and line frequencies.
Color video signals, so-called composite video, blanking and sync signals (CVBS), are essentially composed of a brightness signal or a luminance component (Y), two color difference signals or chrominance components (U, V or I, Q), vertical and horizontal sync signals (VS, HS) and a blanking signal (blank, BL). The structure of a composite video signal (CVBS) and the corresponding Y, U and V signals are shown in FIG. 1.
FIG. 1a shows a composite video signal for an EBU (European Broadcasting Union) color beam test signal, in which the six hue(tint) values belonging to the vertical color beam in "carrier packets" with the color carrier frequency are superimposed on the luminance component Y. For color carrier generation a color subcarrier frequency sync pulse, the burst, is transmitted directly behind the line sync pulse, SYNC. The burst phase and the burst amplitude are used as reference values for determining the line and the color saturation of the demodulated signal which is represented by the individual carrier packets.
The different coding processes NTSC, PAL and SECAM used in the known color television standards differ in the nature of the chrominance transmission, particularly the different systems use different color subcarrier frequencies and different line frequencies.
The following explanations relate to PAL and NTSC systems, but correspondingly apply to other standard video signals and nonstandard signals.
The color subcarrier frequency (fsc) of a PAL system and a NTSC system is ##EQU1##
In addition, in the PAL and NTSC systems, the ratios of the color subcarrier frequency (fsc) to the line frequency (fh) is given by ##EQU2## so that the phase of the color subcarrier in NTSC changes by 180.degree./line and in PAL by 270.degree./line.
The prior art digital video signal processing and decoding differentiates between two system architectures. They are burst-locked architecture and line-locked architecture. These systems operate with sample frequencies for the video signal, which are produced in phase-locked manner to the color subcarrier frequency transmitted with the burst pulse or in phase-locked manner to the line frequency, respectively.
In the case of decoders with a burst-locked architecture the sample frequency is selected in such a way that on the one hand it is not too high in order to keep the power loss low and on the other hand so that the Nyquist theorem is fulfilled, i.e. f.sub.a &gt;2.multidot.fsc. For problem free processing of the modulated color carrier in the decoder it is appropriate to use a sample frequency corresponding to an even multiple of the color subcarrier.
In line-locked architectures the clock of the digital system is derived from the line frequency and is an integral multiple of the latter, so that an integral number of pixels are produced per line.
Both systems suffer from the disadvantage that the clock frequencies for digitizing the video signal are derived from the video system, namely from the color subcarrier frequency or the line frequency, whereas in a PC environment working takes place with completely different clock frequencies, so that due to the different frequencies in the overall system intermodulation products and crosstalk of signals can have a disturbing effect on the overall operation and image quality. As the clock frequencies of PC's are not generally suitable for sampling video signals, because they do not satisfy the above-explained conditions, prior art decoders in each case have their own oscillators for producing the sample frequencies suitable for a particular television standard.
A data stream of a sample signal with a specific desired clock frequency can also be produced in that input sample values digitized with a first predetermined clock frequency are converted into desired sample values at a second virtual sample frequency.
Sample rate conversion and scaling can be looked upon as identical functions, because both are based on the interpolation of a data stream in a clock domain, in order to produce another data stream in another virtual clock domain. Scaling is normally understood to cover a wider range of derived sample rates than sample rate conversion and operates both in the horizontal and vertical direction. Generally said wider range requires an adaptive low-pass filters in order to avoid aliasing.
In digital color video signal decoders the input sample rate converter requirements are particularly high, because it has to transmit a composite video signal (CVBS), whereas the output sample rate converter and/or scaler only has to transmit a single signal component (Y, U, V). From research carried out in connection with the human visual perception, it is known that such composite signals are relatively sensitive to distortions of their high frequency components. The composite signal requires a good restoration of its high frequency contents in order to avoid high frequency distortion, which can occur in the chrominance channel as low frequency signal components following demodulation and in order to avoid interference or "crosstalk" between chrominance and luminance.
The following example illustrates the requirements made with regards to the precision of a sample rate converter for a digital color video signal decoder. On converting sample values digitized with a clock frequency of 30 MHz (PC-clock) into desired sample values at a virtual sample frequency of 17.72 MHz (4.multidot.fsc (PAL)), the time variation between the desired sample values of the virtual sample frequency must be max 1.5 ns and the phase shift max 2.degree., so that the variation of the sample values are not visible as color errors.
Depending on the nodes and with what particular weighting interpolation takes the sample frequency, whereas with an equally weighted interpolation, i.e. with uniform node spacings, amplitude errors occur.